Aquarium cleaning

Hi friends. A friend of mine recently gave me a 29 gal. tank that it
2-3 years old and seems to be in good shape, although there is some of
(what appears to be) mineral buildup in areas of the glass that
scrubbing doesn't seem to remove. Does anybody have a recommendation
as to the best thing to use to remove it? It probably isn't all that
noticeable under water, but it bugs me and I want to start with an
absolutely clean tank.